The present invention relates to an optical transmission system, in particular, relates to such a system which generates high quality, high rate and bandwidth restricted optical signal free from degradation of signal quality in a transmission line.
In an optical transmission system, a large amount of transmission capacity is intended by using high-bit-rate channel, and wavelength division multiplexing. In general, the higher a channel bit rate is, the more severe the effect of chromatic dispersion in an optical fiber is, and possible distance for transmission is shortened in proportional to square of channel bit rate.
In order to decrease the effect of chromatic dispersion due to difference of group velocity depending upon wavelength, the use of bandwidth restricted code such as an optical duobinary transmission system is useful as described in K. Yonenaga and S. Kuwano, IEEE J. Lightwave Technol., Vol. 15, No. 8, 1997.
FIG. 21 shows a block diagram of a prior optical duobinary transmitter. An input binary signal to be transmitted is applied to an input terminal 10, then, to a precoder 32 which effects code conversion, through an inverter 30 which inverts a binary signal. The precoder 32 includes an exclusive OR circuit 32a and a one bit delay circuit 32b as shown in FIG. 21. An output logic signal of the precoder 32 is kept when an input logic signal is 0, and an output logic signal is inverted when an input logic signal is 1. An output of the precoder 32 is applied to a differential distribution circuit 34 which provides a pair of NRZ (non-return to zero) signals in differential form. Each of the pair of NRZ signals is converted to a ternary duobiary signal by a low pass filter 100-1 or 100-2 which has 3 dB cut-off frequency approximate at the ¼ frequency of signal (clock frequency. A filter 100-1 or 100-2 which operates above is called a duobinary filter. An electrical-optical converter 110 is for instance implemented by a Mach Zender intensity modulator (MZ) of dual electrode drive type, having electrical-optical crystal such as Lithium-Niobate (LiNbO3). A pair of duobinary signals generated by duobinary filters 100-1 and 100-2 are applied to electrodes of the MZ modulator after amplification by amplifiers 102-1 and 102-2 up to half wavelength voltage. The numeral 18 is an optical output, and the numeral 36 is a light source of continuous light which is subject to be modulated by the MZ modulator 110.
FIG. 22 shows operation of a MZ modulator. FIG. 22(a) shows waveform of electrical duobinary signal for driving a MZ modulator. A duobinary signal is a ternary signal having three levels +1, 0 and −1 as shown in FIG. 22(b). The transmission factor of a MZ modulator varies sinuously as shown in the optical transmission characteristic in FIG. 22(c) depending upon drive voltage which is voltage difference between two electrodes of the MZ modulator. When two electrodes are complementary driven, an undesired chirp in an output optical signal may be zero in principle. Therefore, when a D.C. bias voltage (B) is set so that the optical transmission factor is the minimum as shown in FIG. 22(c), an optical phase of an optical output switches just when an input voltage crosses the bias voltage (B), and therefore, an optical duobinary signal which has binary intensity waveform is obtained. Although an optical duobinary signal is a binary intensity signal as shown in FIG. 22(d) and FIG. 22(e) in optically modulated form, it is essentially ternary duobinary signal if we consider optical phase (0, π), and has the equivalent bandwidth as that of duobinary signal. Therefore, an optical duobinary signal has the advantages of both binary intensity modulation and ternary duobinary signal, so that demodulation is possible by binary intensity detection, and narrow-band characteristic of a duobinary signal is obtained.
FIG. 23 shows a block diagram of a whole duobinary transmission system including an optical duobinary transmitter 120, a transmission line 124, optical amplifiers 122, and a receive system having an optical-electrical converter 126, a low pass filter 128, a decision circuit 130 and an binary data output terminal 132. An optical duobinary transmitter 120 in FIG. 23 may take the structure as shown for instance in FIG. 21. In a receive side, a signal is demodulated by merely detecting light intensity as is the case of detection of binary intensity modulation signal.
FIG. 24 shows actually measured relations between chromatic dispersion and power penalty for 40 Gbit/s optical duobinary signal (white dot) and 40 Gbit/s binary NRZ intensity modulation signal (black dot). The horizontal axis shows chromatic dispersion value, and the vertical axis shows power penalty for bit error rate (BER) 10−9. A power penalty is defined as the increase of receiver sensitivity compared with that measured at the BER of 10−9 when the chromatic dispersion is 0. A dispersion tolerance is defined so that it is the width of chromatic dispersion value which satisfies the power penalty less than 1 dB. FIG. 23 shows that the dispersion tolerance of an optical duobinary signal is 200 ps/nm, and the dispersion tolerance of a binary NRZ signal is 95 ps/nm, therefore, the former is more than twice as large as the latter. Thus, an optical duobinary signal has the advantage that the restriction by chromatic dispersion is considerably decreased in high rate signal transmission which has severe effect of chromatic dispersion.
However, a prior optical duobinary transmission system has the disadvantage that an optical transmitter is complicated and requests complicated signal process. In particular, an optical duobinary signal which is generated by a duobinary filter must be amplified up to the level which is enough for driving an optical modulator. In general, voltage level requested for driving an optical modulator is several times as high as voltage level for operating a high rate digital integrated circuit. Therefore, an amplifier which drives an optical modulator is operated in the high power region where an output voltage is apt to saturate.
In case of a binary NRZ intensity modulation system, even if a driver amplifier is operated in high power region where saturation begins, no degradation of aperture of eye pattern occurs, or waveform is even shaped by shortening rising time and falling time of waveforms.
However, in case of a ternary duobinary signal, it is essential to keep waveform itself, therefore, a driver amplifier for driving an optical modulator must have fine linearity in gain characteristics.
Thus, if a driver amplifier for driving an optical modulator used in a conventional binary NRZ intensity modulation system is used for amplifying a ternary duobinary signal, a small distortion of waveform is emphasized, and severe inter-symbol interference is generated. Further, it might be possible that an inter-symbol interference is emphasized by reflection between a duobinary filter and an amplifier, and/or reflection between an amplifier and an optical modulator.
FIG. 25(a) shows waveform of duobinary signal (electrical signal) of 40 Gbit/s generated by a prior driver, and FIG. 25(b) shows optical intensity waveforms modulated by said signal. It is noted in FIG. 25 that an electrical signal for driving an optical modulator has asymmetrical pattern in eye apertures between upper eye opening and lower eye opening, and further an optical intensity signal modulated by said electrical signal is degraded in waveforms because of inter-symbol interference although an eye aperture is kept. An inter-symbol interference degrades receive sensitivity of optical duobinary signal, and dispersion tolerance so that distance for transmission is considerably decreased.